1. Field of the Invention
The present invention relates to a solid-state laser and more specifically to a semiconductor-laser-pumped solid-state laser.
2. Description of Related Art
In recent years, there has been active research and development conducted with regard to solid-state lasers with semiconductor laser excitation, and in the electronics and machinery industry, a switch is being made from lamp excitation to semiconductor laser excitation, in such lasers as low-output solid-state lasers.
In the most commonly used type of laser, the Nd:YAG laser (Neodium YAG; wavelength: 1.06 micron), a highly reliable semiconductor laser excited solid-state laser has been developed, relying on high crystal quality and accumulated technology.
However, in the case of exciting an Nd:YAG crystal using a semiconductor laser, while the excitation wavelength is 808 nm, the laser wavelength is 1064 nm, the quantum efficiency, which is the photon energy ratio, is 76%.
The laser optical-to-optical conversion efficiency intrinsically cannot exceed this value. Accompanying this, there is an unavoidable conversion of the 30% of the excitation power (the heat generation ratio) to heat.
For this reason, in the case of high-power excitation, the heat generated in the Nd:YAG crystal causes a rise in temperature, this causing distortion in the crystal.
When excitation is excessive, the thermal stress exceeds the mechanical strength of the crystal, this leading to its destruction. Additionally, the temperature rise within the crystal causes the formation of a thermal lens and thermal birefringence in the crystal, this causing a deterioration of the laser characteristics (efficiency and beam quality).
In general, in a high-output laser having an output of 100 W or greater, these thermally caused phenomena appear prominently, and such characteristics as laser efficiency and beam quality are considerably worse than those of a low-output laser.
Recently, there has been active research and development with regard to solid-state lasers having a wavelength in the 1-micron band which use active ions other than neodium.
Of these, the above-noted quantum efficiency of a Yb (ytterbium) solid-state laser and particularly a Yb:YAG laser is excited by a semiconductor laser having a wavelength in the range from 940 or 970 nm and emitting light at 1030 nm and thus the quantum efficiency of the Yb:YAG laser exceeds 90%.
For this reason, the intrinsic efficiency is high. Additionally, because the heat generation ratio is approximately 10%, scaling to a high output can be simply done to three times that of an Nd:YAG laser.
Other superior features of ytterbium include wide absorption spectrum lines (approximately from 2.5 times to 10 times that of Nd:YAG; note that 18 nm at 940 nm and 4 nm at 970 nm), a large absorption cross section and no up-conversion or excited state absorption.
However, because the laser lower level is a Stark level within the ground-state manifold (quasi-three-level system), as much as 5% of all of the ytterbium ions are in a thermally excited condition in the laser lower level at room temperature.
For this reason, it is difficult to achieve the population inversion that is required for laser oscillation and additionally, because the number of population at the lower laser level exhibits a dependency on temperature, the laser oscillation threshold value and output performance are also dependent upon temperature.
In the past, to achieve high efficiency and high output by reducing the number of population of the lower levels, the crystal was cooled.
For example, in Optics letters, Vol. 16, page 1089 (1991), there is a report of a Yb:YAG laser excited at 941 nm and operated at two points, with the crystal at room temperature (27.degree. C.) and at the temperature of liquid nitrogen (-196.degree. C.).
In Applied Physics B, Vol. 58, page 365 (1994), there is a report of a Yb:YAG laser excited at 940 nm and 970 nm, in which the crystal temperature is varied from -193.degree. C. to +27.degree. C. using a cryostat utilizing the liquid nitrogen.
In both of these reports, a continuously oscillating titanium sapphire laser was used as the excitation light source.
In Optics Letters, Vol. 21, page 480 (1996), there is a report of a Yb:YAG laser which is excited at 940 nm by a semiconductor laser, in which the crystal is held at a temperature of 15.degree. C. using a Peltier element.
And in Trends in Optics and Photonics, Vol. 1, page 12, there is a report of a Yb:YAG laser which is excited at 940 nm by a semiconductor laser, in which the crystal is cooled to -70.degree. C.
The laser characteristics in the case of a semiconductor laser pumped Yb:YAG laser, which is extremely attractive for use in industrial applications are completely different from the above-described case of a titanium sapphire excited Yb:YAG laser.
One completely different aspect is that there is the one in that "change in the absorption efficiency with respect to the excitation semiconductor laser power is caused by a change in the absorption spectrum of the crystal which accompanies a change in the crystal temperature".
With cooling, the absorption line width narrows, thereby reducing the absorption peak value.
For this reason, in a Yb:YAG laser that is excited by a semiconductor laser, the emission spectral width of the semiconductor laser is approximately the same as the absorption line width of the crystal, so that a reduction in spectrum overlap, this causing a prominent change in the absorption efficiency.
The emission spectrum of the continuously oscillating titanium sapphire laser is less than 0.1 nm, so that the absorption efficiency is not caused by the change in absorption line width.
In the past, in a Yb:YAG laser that is pumped by a semiconductor laser that operates either near room temperature or at approximately -70.degree. C., the lower levels population was not sufficiently reduced, leading to the problems of "not being able to obtain the optimum output" and "low optical-to-optical conversion efficiency."
However, in the case of excessively cooling the crystal to the region of -200.degree. C., although the lower level population is sufficiently reduced, the absorption efficiency of the excitation power in a Yb:YAG laser that is pumped by a semiconductor laser decreased, leading to the problems of reduced output and reduced efficiency.
Additionally, cooling to a low temperature such as -200.degree. C. requires a cryostat which uses a medium such as liquid nitrogen. For this reason, for continuous operations of the laser, it is necessary to feed a coolant from outside, making on-site use in production difficult, due to an extreme complicated controlling system therefor and high producing cost.
Additionally, the above-noted problems are exhibited in common in a quasi-three-level laser.